1. Field of the Invention
The present invention relates to an anti-thief security sensor assembly of a type wherein while an infrared beam emitted from an infrared beam projecting unit is constantly received by an infrared beam receiving unit, an alarm is generated when an unauthorized intruder traverses across the path of travel of the infrared beam from the infrared beam projecting unit towards the infrared beam receiving unit.
2. Description of the Prior Art
An anti-thief security sensor assembly enhances its ability of detecting an intruder by arranging an infrared beam projecting unit and an infrared beam receiving unit at positions spaced an appropriate distance from each other with their optical axes aligned with each other. On the other hand, the projecting power of the infrared beam projecting unit is made larger so that the intruder can be detected even if the distance between the infrared beam projecting unit and the infrared beam receiving unit becomes more remote. Due to the fact, when the distance between the infrared beam projecting unit and the infrared beam receiving unit installed is short, the amount of the beam received by the infrared beam receiving unit becomes excessive, causing an AGC (Automatic Gain Adjusting Circuit) to fail in the adjustment of the received beam level, whereby a detection signal level becomes saturated. In such case, the amount of the beam reflective on building walls adjacent the sensor assembly becomes adversely influential in detecting an incident beam amount attenuated by the interception of the intruder, or the interception of the intruder. As a result, the detection of the intruder tends to fail.